docs: def dirs(self, before=time.time(), exited=True):
          """
          Provider a generator of container state directories.
  
          If exited is None, all are returned. If it is False, unexited
          containers are returned. If it is True, only exited containers are
          returned.
          """
          timestamp = iso(before)
          root = os.path.join(self.root, "start-time")
          os.chdir(root)
          by_t = (d for d in glob.iglob("????-??-??T*.*Z") if d < timestamp)
          if exited is None:
              def predicate(directory):
                  return True
          else:
              def predicate(directory):
                  exit = os.path.join(directory, "exit")
                  return os.path.exists(exit) is exited
          return (os.path.join(root, d) for d in by_t if predicate(d))